This invention relates to cathode ray tube construction and more particularly to a high resistive electrical conductive coating employed for suppressing deleterious arcing therein.
The advancement of cathode ray tube technology has resulted in marked improvements in both tube construction and the operational considerations relating thereto, including a trend toward the utilization of higher screen potentials along with the miniaturization and compaction of associated electron gun structures encompassed within the envelope neck portions of smaller diameters. Consequently, spacings between related electrode components in the electron gun structure of the tube have been reduced in keeping with advanced design parameters. The minuteness of these interelectrode spacings, in conjunction with the high voltage differential existant within the tube, and the presence of possible contaminants, increases the probability of dielectric breakdown within the tube structure.
It has been conventional practice in cathode ray tube construction to apply an electrical conductive coating on the interior surface of the funnel member of the tube envelope in a manner to extend from substantially the vicinity of the cathodoluminescent screen into the forward region of the adjoining neck member. This coating, which usually has a high positive electrical potential applied thereto, via connective means traversing the wall of the funnel member, serves as a connective medium conveying a high electrical potential of substantially a common value to both the screen and the terminal electrode of the electron gun assembly oriented within the neck member of the tube envelope. Thus, the condition is present for the possible generation of a spark discharge between the terminal electrode and the adjacent lower voltage electrode in the gun assembly, especially in the presence of aggravating elements such as sublimation deposits, foreign particles, and minute projections extending into the inter-electrode spacings. While considerable effort is expended during tube manufacturing to minimize the factors contributing to dielectric breakdown, the utilization of anode potentials in the order of 30 KV and higher makes the possible presence of contributable arcing conditions factors of extreme importance. Arcing or dielectric breakdown within the cathode ray tube has always been an undesired probability, the magnitude of which has been found to sometimes exhibit destructive intensities of 100 amperes or more. With the increased employment of solid state components in television and allied display devices, arcing within the cathode ray tube can produce catastrophic effects on the vulnerable components in the externally associated operating circuitry. Additionally, an arc discharge initiated within the tube may seriously damage the internal structure thereof and resultantly promote leakage through the sublimation of deleterious metallic deposits on related surfaces in the region of the gun structure.
Cleanliness, precision, vigilance and care in the tube manufacturing process are ever continuing procedures employed to combat the materializing of conditions conductive for arcing. Nevertheless, human factors, processing sublimates, manufacturing tolerances and procedural variations may combine to produce an undesirable and aggravative situation. The discrete use of high resistance coatings on defined interior areas of the funnel member of the envelope has been tried. For example, one such technique is that disclosed by A. V. de Vere Krause in U.S. Pat. No. 2,829,292, wherein a band of resistive coating is internally applied to substantially the juncture region of the funnel and neck members of the tube envelope whereat the snubbers on the terminal electrode of the electron generating assembly make plural-point contact with the high resistance arcing to limit the spark discharge current in the region of the electron gun. However, it has been found in high anode potential tubes that the assembly snubbers tend to effect high resistance point contact with the resistive coating, a condition which is prone to produce intense heat during tube processing when a high voltage conditioning potential of 40 KV or more may be applied to the anode. Such localized heating may cause a buildup of deleterious field emission, ionization and ultimate rupture or checking of the glass wall of the neck member. Additionally, difficulties have been encountered in achieving high resistive electrical conductive coatings that evince uniformity, consistently exhibit the desired electrical characteristics and manifest the necessary tenacious bonding to the surface of the envelope. Since the minimization and eliminating of arcing in present-day color cathode ray tubes is assuming ever increasing importance, it is a prime concern in tube manufacturing to achieve an expedient and consistent coating means for adequately controlling the probable arcing environment within the cathode ray tube per se.